Why does my model duplicate itself in pose mode
I had the same problem with deformations producing duplicates. I figured it was related to the fact that I had added additional bones to the armature post the Ctrl + P > Set Parent > Armature Deform action and the new bones were interfering with the deformation since the weights had not been assigned (@David's comment was the clue for me).

To solve this issue you need to go to Object Mode, select the Mesh, then select the armature and redo the Ctrl + P > Set Parent > Armature Deform action. I used the Automatic Weights option as before and Bam!

The deformations in pose mode are back to normal.